The invention relates to a process for the manufacture of a composite molded structure. It also relates to the composite molded structures thus obtained.
A "composite molded structure" means a molded structure containing:
members, especially for stiffening and/or strengthening, which are either in the form of threads or in the form of sheets or even in the form of plates;
protective members;
various mechanical members such as, for example, those for damping, reducing weight, etc.
optionally, decorative members.
There exist a number of industrial processes capable of producing the assembly of these structures. There may be mentioned:
The so-called "wet route" process, is described, for example, in document FR-A-1,124,600, corresponds to a stacking of freshly adhesive-coated members. This process is nevertheless found to be lengthy to implement, difficult to automate and additionally, requires precautions in use.
The "dry route" adhesive bonding process empoys pre-impregnated members or dry films. This process is found to be easier to automate and above all less boiling. However, this process is found to be difficult to use, bearing in mind the very strict conditions of implementation which are inherent in the specifics of prepregs. In addition, in using the so-called "dry route" process it is difficult to obtain "surface on surface" adhesive bondings in which the adhesive joint is free from microbubbles which tend to weaken the structure obtained.
The assembly process using in-situ injection of resin (expansive or otherwise), is described, for example, in the document FR-A-2,130,758.
Whatever the industrial process employed, it is easy to produce so-called "sandwich" structures with a section of rectangular shape. The constituent members of such a structure are easily positioned either by stacking or by pressing on supports produced for this purpose.
In contrast, when it is desired to produce more complex, nonplanar shapes, or ones exhibiting changes in shape along the line of the product in question, a number of solutions are known at present.
First of all, the use of shaped shells, in which the various members precoated with adhesive are arranged, or into which the resins are injected. This process is found to be costly, bearing in mind the additional operations of producing the shell.
It is also known to employ either the pressure produced by the expansion of polyurethane (see, for example, the document U.S. Pat. No. 4,590,023), or the pressure of air inside inflatable tubes (see, for example, the document CH-A-369,894) to obtain a laying of the casing members flat against the inner walls of the shaped mold. However, in the former case, a free space must be arranged in the mold, for example, by means of spacers, the space intended to be filled up by the expanded polyurethane foam, giving rise to an increased complexity in the arrangement of the molds. Moreover, the casing members can, as already stated, be preimpregnated with a resin capable of interacting with the polyurethane and hence impairing the properties sought after in the structure obtained. Finally, in some cases the polyurethane foam acts as an adhesive bonding between the lower and upper casing members of the structures. It has been found that this adhesive bonding nevertheless remained limited in effectiveness and could be employed only for some applications.
The second case, namely the use of air injected into bags, is employed above all with the aim of lightening the structure. Consequently, hollow cellular objects without a core are obtained, whose behavior and stiffness are sometimes incompatible with some applications of such structures.
Lastly, it is always possible to obtain special shapes by machining. This machining uses up material, is found to be costly in view of this additional operation, and does not enable all the desired structures to be obtained. What is more, it can affect the unity and continuity of the structure and hence the related physical properties. Furthermore, the production of a decoration is made more difficult, because this process rules out the possibility of a flat predecoration.